This invention relates generally to switches for optical networks and in particular to optical switching fabrics with tilting mirrors.
As optical fiber progressively supplements and replaces metal wire as the backbone of telecommunications networks, the switches that route optical signals have emerged as a significant bottleneck. Transmission systems move information as optical photons but the switching systems and so-called crossconnect fabrics that switch, route, multiplex, and demultiplex optical signals have generally been electronic. Electronic switching requires light to be converted to an electronic signal to pass through the switch and then be reconverted to light in a process termed optical-electronic-optical (OEO) conversion that introduces both time delay and cost.
There is great interest in the telecommunications industry, therefore, in developing all optical switching to avoid the necessity of multiple OEO conversions. On long haul networks, ten""s or hundred""s of individual wavelengths, each carrying a signal, are multiplexed onto each fiber. Switches are desired that provide all optical switching at the fiber level, the wavelength level, or at both levels. As described, for example, by Bishop et al. in Scientific American (January, 2001, pp 88-94), all optical switches based on a number of underlying technologies including Micro Electro Mechanical Systems (MEMS) tilting mirrors, thermo-optical devices, bubbles formed by inkjet printing heads, and liquid crystals, have been proposed. Optical fiber switches based on MEMS mirrors are particularly attractive because they can incorporate very large scale integrated circuits and can be robust, long-lived, and scalable.
An optical fiber switch described in U.S. Pat. No. 5,960,132 to Lin, for example, includes an array of hinged MEMS mirrors, each of which can be rotated about its hinge to reflect or not reflect light in a certain direction. An array of N2 such mirrors is required to switch signals carried by N input optical fibers from one to another of N output optical fibers. Unfortunately, N2 scaling results in unmanageably complex systems for large N.
Another optical fiber switch described in Bishop et al., cited above, as well as in Bishop et al., Photonics Spectra (March 2000, pp. 167-169) includes an array of MEMS mirrors disposed on a single surface. Each mirror tilts independently to direct light received from an array of input/output optical fibers via a folding flat to any other mirror and thus to any input/output fiber. No internal optical diagnostics for this switch have been described in publications to date.
Still other optical fiber switches are based on two arrays of MEMS mirrors that can be tilted in any direction. Incoming light is directed onto a mirror in the first array which deflects it onto a predetermined mirror in the second array. The mirror in the second array, in turn, directs the lights to the predetermined output port. In these so-called, 2N configurations, the position of the mirrors has to be controlled very precisely, to small fractions of degrees to provide the desired connections.
Optical fiber switches having a low insertion loss and that can be finely tuned to cross-connect large numbers of input and output fibers would further the development of fiber optic telecommunications networks.
An optical switching fabric is an optical switch with multiple input ports and multiple output ports that allows an optical signal entering the device on any input port to be directed to any output port. Optical switching fabrics according to embodiments of the present invention include sensing and monitoring devices that enable precise initial calibration and continuous switch connection status monitoring and control.
The present optical switching fabrics include, therefore, multiple input ports, multiple output ports, a first set of multiple mirrors disposed on a first surface, typically in the form of an array, a second set of multiple mirrors disposed on a second surface, typically in the form of an array, and a dichroic beamsplitter. Each one of the first set of mirrors is individually controllable to direct light from a corresponding one of the input ports to any one of the second set of mirrors, via the dichroic beamsplitter. Each one of the second set of mirrors is individually controllable to direct light, incident on it from one of the mirrors in the first set of mirrors, to a corresponding one of the output ports.
Switching fabrics according to an embodiment of the present invention further include control light sources, which provide light beams separate from the signal carrying light beams, and position sensing detectors, which enable the positions of the mirrors to be detected and controlled when no signal light is present in the switching fabric. The control devices include a first control light source located to illuminate the first set of mirrors and a first position sensing detector located to detect light from the first control light source that has been reflected by the first set of mirrors. The signals provided by the first position sensing detector correspond to the positions of the first set of mirrors.
In addition, such switching fabrics include a second control light source positioned to illuminate the second set of mirrors and a second position sensing detector positioned to detect the reflected control light such that signals from the second position sensing detector correspond to the positions of the second set of mirrors. A third position sensing detector may be positioned to detect control light that has been reflected by the first set of mirrors and the second set of mirrors. The control light sources provide light at a different wavelength than the wavelength of the optical signals directed by the switching fabric.
According to another aspect of the present invention, the switching fabric includes a first sensor positioned to detect the intensity of light entering the switching fabric from the input ports. A beamsplitting cube may be included in the optical path between the input ports and the first set of mirrors to deflect a small portion of the input light to the first sensor. A second sensor located to detect the intensity of light backscattered from the output ports is also included. When the output light is properly aligned on the output ports, the backscattered light is at a minimum. Infrared cameras are useful as the first and second sensors. By including suitably oriented optical reflectors, the intensity of light from the input ports that has been reflected by the first set of mirrors and by the second set of mirrors can be detected on the second sensor and the intensity of backscattered light that has been deflected by the second set of mirrors and then by the first set of mirrors can be detected on the first sensor. The optical switching fabric may also be controlled according to the signals from these two sensors.
According to yet another aspect of the present invention, a subset of the input ports and a subset of the output ports are dedicated as monitor channels. The switching fabric is configured to direct light from the monitor input ports to the monitor output ports via the first set of mirrors and the second set of mirrors. One or more monitor light sources that emit light at a wavelength similar to the wavelengths of the optical signals controlled by the switching fabric are attached to the monitor input channels and one or more monitor detectors are attached to the monitor output ports. The monitor source(s) and detector(s) provide continuous information about the status of the switching fabric without the need for signal wavelengths to be present. Further, one or more light sources can be combined with one or more detectors at both the monitor input ports at the monitor output ports to also detect the passage of light through the system in the opposite direction from the output ports to the input ports.
The optical switching fabric is controlled by an optical switching fabric controller using alignment look-up tables that are determined during an initial factory calibration process. Periodically, the alignment look-up tables can be recalibrated using the monitor light sources and detectors. A calibration correction to all channels through the switching fabric can be computed from the correction determined for the monitor channels.